Jesus Christ is the one who came. He came with water and with blood. He did not come by water only. No, Jesus came by both water and blood. And the Spirit tells us that this is true. The Spirit is the truth.
We believe people when they say something is true. But what God says is more important. And this is what God told us: He told us the truth about his own Son.
Whoever believes in the Son of God has the truth that God told us. Whoever does not believe God makes God a liar, because they do not believe what God told us about his Son.
We can come to God with no doubts. This means that when we ask God for things (and those things agree with what God wants for us), God cares about what we say.
Suppose you see your fellow believer sinning (sin that does not lead to eternal death). You should pray for them. Then God will give them life. I am talking about people whose sin does not lead to eternal death. There is sin that leads to death. I don't mean that you should pray about that kind of sin.
And we know that the Son of God has come and has given us understanding. So now we can know the one who is true, and we live in that true God. We are in his Son, Jesus Christ. He is the true God, and he is eternal life.